The invention relates to improvements in apparatus for the comminution and grinding of brittle grinding stock such as raw cement meal, cement clinker, ore, coal or the like, and particularly comminution of material which is damp before grinding.
What is referred to as a "tandem grinding-drying system" ("Cement-Data-Book" by Duda, Second Edition, 1977, page 101) has been disclosed for grinding and simultaneous drying of damp material for the manufacture of raw cement meal wherein a tube mill i.e., a ball mill is preceded by an impact hammer mill and both mills are traversed by a hot gas stream. Raw material up to 100 mm particle size and up to about 12% moisture can be ground into raw cement meal extremely well and dried with this known system upon employment of hot exhaust gas from the raw cement meal pre-heater. However, the energy exploitation of the ball mill thereby utilized is not very satisfactory.
Considerable efforts have been undertaken in order to enhance the low energy exploitation in the comminution machines, particularly in the tube mills. Thus, a two-stage means for comminution and grinding of brittle grinding stock has been disclosed (European Patent Application No. 0 084 383) wherein the non-pre-comminuted grinding stock, such as, lumps of cement clinker, are first pressed under high pressure in the nip of a high-pressure roll press in the first stage. This partly leads to particle destruction, partly due to the production of incipient cracks in the interior of the particles and being visibly expressed in the formation of agglomerates which are then disagglomerated or destroyed in the second stage in a tube mill or a ball mill with comparatively low energy outlay and can then be ground to finished product fineness. In the material pressing, a roll press operated with a high pressing power of more than 2 t/cm of roller length produces agglomerates whose grindability is considerably improved compared to unpressed material, so that the two-stage comminution leads overall to a noticeable reduction of the specific energy requirement. In the roll press, the individual particles of material are mutually crushed in a product bed, i.e., in a material fill compressed between two surfaces, so that what is referred to as product bed comminution occurs in the first stage.
With known grinding means preceding a high-pressure roller press, this can be non-uniformly loaded when the grain size distribution of the feed stock is extremely non-uniform, this being the case with initial material for raw cement meal. The grain size of this initial material can fluctuate within a range from 0 to more than 50 mm and above. The risk is present that the high-pressure roll press will run erratically. The initial material for raw cement meal is usually damp, having an initial moisture up to about 15%, and since this material must not only be ground, but must also be dried, there is also a need to utilize a high-pressure roll press with product bed comminution in a grinding-drying operation, and this has not been proposed or realized previously.
Given a comminution or grinding apparatus of the type referred to, an object of the invention is to insure that the high-pressure roll press utilized receives a feed stock having a somewhat uniform grain size distribution and that the high-pressure roll press with product bed comminution can be integrated in a grinding-drying of damp material in a meaningful way.
Employing a two-stage comminution and grinding in accordance with the invention using a high-pressure roll press or roll-jaw crusher with a further comminution machine, particularly an impact hammer mill, the two comminution machines are interconnected to form a circulating grinding system via a classifier, and particularly a sifter. The fresh initial material to be comminuted is not delivered to the high-pressure roll press, but to the other comminution machine, preferably an impact hammer mill, which can comminute feed stock having a grain size distribution from 0 through about 80 mm. This is followed by the high-pressure roll press. A sifter is inserted between the impact hammer mill and the roll press, the sifter separating the adequately fine fraction (about 20 through 30%) already contained therein from the product discharge of the impact hammer mill as finished product. Only coarse sifter material (grits) proceed to the product admission of the high-pressure roll press, the grain size distribution thereof being uniform and the absolute grain size being small in comparison to the fresh initial material which is supplied to the impact hammer mill. In the comminution and grinding apparatus of the invention, therefore, the high-pressure roll press can be operated free of sudden load, being always uniformly operated with an optimally set roll pressing power of, for example, 6-9 t/cm of roller length. The product discharge of the roll press is in communication with the product admission and/or with the product discharge of the impact hammer mill. When the roll press discharge, which can be composed of agglomerates, is supplied to the product admission of the hammer mill together with the fresh initial material, then the easily crumbling agglomerates of the press are disagglomerated by the impact hammer mill without additional cost expenditure. When the roll press discharge by-passes the impact hammer mill and is introduced into the product discharge thereof, then the product agglomerates are disagglomerated in the rising main arranged between impact hammer mill and sifter and/or in a corresponding product admission sluice such as, for example, a tube worm.
The invention can be particularly well applied in grinding-drying of fresh, damp material, such as initial material for raw cement meal. Using the grinding-drying installation of the invention, a hot gas conduit is connected to the product admission of the impact hammer mill equipped with a closed bottom and a rising main is arranged between the product discharge of the impact hammer mill and the sifter being fashioned as a flow dryer. The coarse sifter product (grits) returned from the sifter to the high-pressure roll press has been dried to such a great degree that the product bed comminution of this coarse sifter material in the nip of the roll press is not impeded by the slight residual moisture which may still be present. Especially characteristic of the grinding-drying system of the invention for comminution and grinding of damp material is that the tube mill or ball mill which had hitherto always been utilized in grinding-drying systems and which is distinguished by an extremely poor energy exploitation is no longer present. It is replaced by a high-pressure roll press operated in accord with the principle of product bed comminution. This results in a considerable reduction of the capital costs as well and primarily in a reduction of the specific energy requirement of the overall grinding-drying system. It is also easily possible to remodel existing grinding-drying systems into the grinding-drying system of the invention after elimination of the tube mill or ball mill in order to benefit from the extremely high energy savings during operation of the grinding-drying system of the invention.